Liquid crystal polyester fiber, method for producing the same, and use of the same

ABSTRACT

The present invention provides a method for producing a liquid crystal polyester fiber, the method comprising a step of spinning a liquid crystal polyester that has a glass transition temperature and a melting point, both observed by differential scanning calorimetry, and that has a flow initiation temperature of 250° C. or higher, followed by a heat treatment at 230° C. or lower. Even though the heat treatment is conducted at a low temperature, the obtained liquid crystal polyester fiber has a high strength.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal polyester fiber, a method for producing the liquid crystal polyester fiber, and use of the same.

2. Description of the Related Art

A liquid crystal polyester fiber has been attracting attention as a fiber having high strength and high modulus of elasticity, and a number of studies thereon have been made in recent years. Since a liquid crystal polyester exhibits anisotropy in a molten state, a liquid crystal polyester fiber having relatively high strength can be obtained without drawing by optimizing extrusion conditions through a cap nozzle when the liquid crystal polyester is subjected to melt spinning. In order to obtain a liquid crystal polyester fiber having a higher strength, a heat treatment is sometimes carried out under high temperature conditions.

This is because the heat treatment allows the liquid crystal polyester to be highly polymerized and highly crystallized while allowing the polyester to maintain a fiber state, thus leading to an improvement in the strength. However, when the liquid crystal polyester fiber is subjected to a heat treatment at high temperature, agglutination of single yarns occurs so that single yarn splittability may be deteriorated (see, Japanese Published Patent Publication No. 58-502227). Further, Japanese Patent Application Laid-Open No. 3-260114 proposes a method in which a fiber made from a liquid crystal polymer is subjected to a heat treatment under tension of 1 g/d or higher. Japanese Patent Application Laid-Open No. 5-222611 proposes a method in which a liquid crystal polyester fiber is accumulated in a loop form and the fiber accumulation is subjected to a heat treatment by passing through a heat treatment furnace.

However, the methods of producing a liquid crystal polyester fiber, which have hitherto been proposed, require high temperature conditions, resulting in enormous energy consumption and not necessarily satisfactory productivity. Thus, there has never been found a method of producing a liquid crystal polyester fiber, which is excellent in single yarn splittability by a heat treatment at low temperature.

SUMMARY OF THE INVENTION

Under such circumstances, the present inventors found a method for producing a liquid crystal polyester fiber having a high strength, the method comprising a step of heat treatment at a lower temperature in a shorter time than in the conventional production methods of the liquid crystal polyester fiber. This led to completion of the present invention.

Thus, the present invention provides a method for producing a liquid crystal polyester fiber, the method comprising a step of spinning a liquid crystal polyester that satisfies the following features (a) and (b):

(a) a glass transition temperature and a melting point are observed by differential scanning calorimetry, and

(b) a flow initiation temperature is 250° C. or higher, which is followed by a heat treatment at 230° C. or lower.

In the present invention, a liquid crystal polyester fiber having a high strength can be produced by a heat treatment at a lower temperature in a shorter time when compared with conventional methods for producing a liquid crystal polyester fiber. Since the heat treatment in the present invention is carried out at low temperature, it tends to easily obtain a liquid crystal polyester fiber having excellent single yarn splittability without causing agglutination of single yarns in the fiber.

The liquid crystal polyester fiber obtained by the present invention can be used for various industrial applications, and is extremely useful from an industrial point of view.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph schematically showing a calorimetric profile of a DSC chart of a typical liquid crystal polyester; and

FIG. 2 is a graph schematically showing an essential part of analysis means for determining Tg from a Tg pattern.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, a liquid crystal polyester fiber with a high strength can be produced by a method comprising a step of spinning a liquid crystal polyester, followed by a heat treatment at low temperature.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings as needed.

<Liquid Crystal Polyester>

First, a liquid crystal polyester used in the present invention will be described below.

The liquid crystal polyester used in the present invention is a polyester called a thermotropic liquid crystal polymer which forms a melt that exhibits optical anisotropy at a temperature of 450° C. or lower.

The liquid crystal polyester used in the present invention includes not only a form in which aromatic groups, or an aromatic group and an aliphatic group are bonded through an ester group (—COO— or —OCO—) as a bonding group, such as an aromatic polyester or an aromatic-aliphatic polyester, but also aromatic poly(ester-amide) having an amide group (—CONH— or —NHCO—) at a portion of the bonding group and aromatic polyester-carbonate having a carbonate ester group (—OCOO—) at a portion of the bonding group.

In order to satisfy the above-described features (a) and (b), it is preferred that the structural units connected through bonding groups, including an ester group, are aromatic groups, and it is more preferred that the aromatic groups account for 50% by mole or more of the entire units, further preferably 80% by mole or more, and all-aromatic polyester, all-aromatic poly(ester-amide) or all-aromatic polyester-carbonate are particularly preferred, in which substantially all units are aromatic groups. It is preferred that the aromatic groups are monocyclic aromatic groups, and a preferred liquid crystal polyester will be described later.

In the present invention, a liquid crystal polyester that satisfies features (a) and (b) (described below) is utilized. The feature (a):

The liquid crystal polyester has a glass transition temperature and a melting point, both of which are observed by differential scanning calorimetry (hereinafter, referred to as “DSC measurement”). The DSC measurement with regard to the feature (a) refers to a calorimetry that is performed under a nitrogen atmosphere at a heating rate of 10° C./min from room temperature (about 23° C.) to 400° C. The liquid crystal polyester satisfying the feature (a) has an endothermic pattern (hereinafter, referred to as a “Tg pattern”) showing the glass transition temperature (hereinafter, referred to as “Tg”) and an endothermic peak (hereinafter referred to as a “Tm peak”) showing the melting point (hereinafter, referred to as “Tm”), both of which are observed in the calorimetric profile of the DSC chart measured.

The Tg pattern and the Tm peak will be described with reference to FIG. 1. FIG. 1 is a graph schematically showing a DSC chart of a typical liquid crystal polyester, in which the x-axis represents a temperature (increasing toward right) and the y-axis represents a temperature change showing endo/exotherm. In the calorimetric profile of the liquid crystal polyester, a “stepwise change” is first observed as raising the temperature from room temperature, wherein endotherm occurs based on Tg. The stepwise changing emdothermic pattern is referred to as a Tg pattern in the present invention. Tg is sought from the Tg pattern generally by analysis means in accordance with JIS K7121 (1987) “Method for Measurement of Transition Temperature of Plastic”. It is explained briefly with reference to FIG. 2 (enlarged schematic diagram of a Tg pattern). First, two auxiliary lines L1 and L2 are drawn by extending a base line on a high temperature side (exothermic side) and a base line on a low temperature side (endothermic side) in the Tg pattern. Nexti an endothermic change between L1 and L2 (ΔTh) is sought. Then, a half value of the ΔTh (½ΔTh) is sought, and L2 is shifted to the exothermic side by ½ΔTh, so as to draw a third auxiliary line (Lh) parallel to L2. Then, the temperature value where Lh and the calorimetric profile intersects is defined as Tg. The Tm peak based on Tm is an endothermic peak, which is observed on the higher temperature side than the Tg pattern. The temperature value at the apex of the Tm peak is Tm. The endothermic change in the Tg pattern and the endothermic change in the Tm peak in the DSC measurement are determined by a parameter called an SN ratio in the DSC measurement. The SN ratio denotes a ratio of a calorimetric difference of a signal component to a calorimetric difference of a noise component in the DSC measurement. The Tg pattern is defined to be observed when a pattern of endothermic change with an SN ratio of 30 or more (stepwise change) is seen in the endothermic direction in the DSC measurement. In contrast, the Tm peak is defined to be observed when a peak with the SN ratio of 50 or more is seen in the endothermic direction on a higher temperature side than the Tg pattern.

Tg of the liquid crystal polyester used in the present invention is generally observed in a range of from 80° C. or higher to lower than 250° C., but the liquid crystal polyester having Tg of 150° C. or lower is particularly preferred for use in the present invention.

In contrast, the Tm peak is observed at a higher temperature side than the Tg pattern, as described above. In the present invention, a liquid crystal polyester preferably has a peak (Tm) of 250° C. or higher, and more preferably 280° C. or higher.

The Feature (b):

The flow initiation temperature in the feature (b) is an indicator which represents a molecular weight of a liquid crystal polyester known in the art (see “Synthesis, Molding and Application of Liquid Crystalline Polymer” edited by Naoyuki Koide, pp. 95-105, CMC, published on Jun. 5, 1987). The flow initiation temperature represents a temperature when a melt viscosity of 4,800 Pa·s (48,000 poise) is obtained as a result of the following procedure of processing a liquid crystal polyester into a powder, charging the powder in a capillary type rheometer equipped with a die measuring 1 mm in inner diameter and 10 mm in length, applying a load of 9.8 MPa (100 kg/cm²), and extruding the liquid crystal polyester at a heating rate of 4° C./min while measuring the melt viscosity using a flow tester. The liquid crystal polyester used in the present invention requires the flow initiation temperature to be 250° C. or higher, in order to improve a spinning property when the liquid crystal polyester is melt-spun to obtain a liquid crystal polyester in a fiber form before carrying out a heat treatment, in the following production method of the liquid crystal polyester fiber. When the flow initiation temperature is lower than 250° C., thread breakage occurs frequently and stable spinning tends to be relatively difficult, even though countermeasures such as increasing the discharging volume upon melt-spinning are taken.

Thus, the liquid crystal polyester satisfying the features (a) and (b) enables realization of a liquid crystal polyester fiber having a high strength by the following heat treatment at a low temperature in a short time.

In order to enhance the strength of the liquid crystal polyester fiber by a heat treatment under a lower temperature condition, it is preferred that Tg is lower, and it is more preferred to use the liquid crystal polyester having Tg of 150° C. or lower as mentioned above. Further, when the heat treatment is carried out at around the melting point, single yarns tend to cause agglutination. Therefore, it is preferred that the liquid crystal polyester exhibiting Tm on the higher temperature side, and that the temperature difference between Tg and Tm be 100° C. or larger. When the temperature difference is 100° C. or larger, agglutination of single yarns are prevented adequately by the following preferable method for producing a liquid crystal polyester, which makes it advantageous to obtain a liquid crystal polyester fiber having a high strength.

Next, a preferred liquid crystal polyester used in the present invention will be explained in detail.

In view of the feature (b), it is preferred to use an aromatic liquid crystal polyester mainly having aromatic groups as its structural units. In view of satisfying the feature (a), it is preferred to use an aromatic liquid crystal polyester having a segment relatively rich in flexibility so as to exhibit Tg, and a segment whose crystallinity is relatively high so as to exhibit Tm, in combination.

Example of the aromatic liquid crystal polyester include a liquid crystal polyester having a monomeric unit derived from aromatic hydroxycarboxylic acid (hereinafter, referred to as “hydroxycarboxylic acid unit”), a monomeric unit derived from aromatic diol (hereinafter, referred to as “diol unit”) and a monomeric unit derived from aromatic dicarboxylic acid (hereinafter, referred to as “dicarboxylic acid unit”); and a liquid crystal polyester having two of more kinds of hydroxycarboxylic acid units. In view of the feature (a), the former liquid crystal polyester is preferred. It is noted that a “monomeric unit derived from” a specific monomer means a structural unit provided by polymerization using the specific monomer.

Examples of preferable monomer providing the unit in the aromatic polyesters include an aromatic hydroxycarboxylic acid, an aromatic diol and an aromatic dicarboxylic acid, as described above.

Specific examples of the hydroxycarboxylic acid include p-hydroxybenzoic acid, m-hydroxybenzoic acid, 2-hydroxy-6-naphthoic acid, 2-hydroxy-3-naphthoic acid, 1-hydroxy-4-naphthoic acid, 4-hydroxy-4′-carboxydiphenyl ether, 2,6-dichloro-p-hydroxybenzoic acid, 2-chloro-p-hydroxybenzoic acid, 2,6-difluoro-p-hydroxybenzoic acid, and 4-hydroxy-4′-biphenylcarboxylic acid. These may be singly used, or two or more kinds of them may be used in combination.

Among these, the hydroxycarboxylic acids having such monocyclic aromatic ring as p-hydroxybenzoic acid and such condensed aromatic ring as 2-hydroxy-6-naphthoic acid are preferred since the resulting liquid-crystalline polymer tends to have a higher strength, and such hydroxycarboxylic acids are readily available.

Specific examples of the aromatic diols include hydroquinone, resorcin, methylhydroquinone, chlorohydroquinone, nitrohydroquinone, 4,4′-dihydroxybiphenyl, 1,4-dihydroxynaphthalene, 1, 5-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 2, 6-dihydroxynaphthalene, 2, 7-dihydroxynaphthalene, and such aromatic diols (in which two aromatic groups may be conjugated) as bis-(4-hydroxyphenyl)ketone, bis-(4-hydroxy-3,5-dimethylphenyl)ketone, bis-(4-hydroxy-3,5-dichlorophenyl)ketone, bis-(4-hydroxyphenyl)ether, bis-(4-hydroxyphenyl) sulfide and bis-(4-hydroxyphenyl)sulfone.

Also, examples of the aromatic diols include such aromatic diols (in which two aromatic groups are linked by an aliphatic group) as 2,2-bis(4-hydroxyphenyl)propane, 2,2-bis(4-hydroxy-3,5-dimethyphenyl)propane, 2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 2,2-bis(4-hydroxy-3-chlorophenyl)propane, bis-(4-hydroxyphenyl)methane, bis-(4-hydroxy-3,5-dimethylphenyl)methane, bis-(4-hydroxy-3,5-dichlorophenyl)methane, bis-(4-hydroxy-3,5-dibromophenyl)methane, bis-(4-hydroxy-3-methylphenyl)methane, bis-(4-hydroxy-3-chlorophenyl)methane and 1,1-bis(4-hydroxyphenyl)cyclohexane. By using such an aromatic diol in which two aromatic groups are linked by an aliphatic group, a liquid crystal polyester into which aliphatic groups are introduced can be obtained.

The aromatic diols maybe singly used, or two or more kinds of them may be used in combination. Among these, the aromatic diols having such monocyclic aromatic ring as 4,4′-dihydroxybiphenyl, hydroquinone and resorcin and such condensed aromatic ring as 2,6-dihydroxynaphthalene are preferred, since the resulting first liquid-crystalline polymer tends to have a higher strength, and such aromatic diols are readily available.

Specific examples of the aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarbocylic acid, 1,5-naphthalenedicarboxylic acid, 4,4′-biphenyldicarboxylic acid, methylterephthalic acid, methylisophthalic acid; and diphenylether-4,4′-dicarboxylic acid and 2,2′-diphenylpropane-4,4′-dicarboxylic acid. These may be singly used, or two or more kinds of them may be used in combination.

Among these, the aromatic dicarboxylic acids having such monocyclic aromatic ring as terephthalic acid, isophthalic acid, phthalic acid and such condensed aromatic ring as 2,6-naphthalenedicarbocylic acid are preferred, since the resulting first liquid-crystalline polymer tends to have the features (a) and have easily controlled Tg and Tm, and such aromatic dicarboxylic acids are readily available.

When the liquid crystal polyester is prepared by using an aromatic hydroxycarboxylic acid or aromatic diol whose phenol-based hydroxyl groups are substituted with amino groups, that is, an aromatic aminocarboxylic acid or aromatic amine having phenol-based hydroxyl groups (for example, 4-aminophenol), as a part of monomers, as exemplified above, the resultant polymer is liquid crystal poly(ester-amide). In the case where diphenyl carbonate or the like is used as a reaction reagent when the liquid crystal polyester is prepared, liquid crystal polyester-carbonate whose bonding groups are partly carbonate ester groups is obtained. However, when the liquid crystal polyester has an amino group or a carbonate ester group as the bonding group, it may turn out to be a liquid crystal polyester with which Tm required in the feature (a) is not observed. In this case, the amino groups or carbonate ester groups may be introduced, and the amount of amino groups or carbonate ester groups may be adjusted to the extent that allows Tm to be observed. From this perspective as well, the liquid crystal polyester used in the present invention is preferably an all-aromatic liquid crystal polyester, in which the bonding groups are substantially ester groups alone, and the units are substantially aromatic groups.

Next, the preferred liquid crystal polyester, i.e., an aromatic liquid crystal polyester including an aromatic hydroxycarboxylic acid unit, an aromatic diol unit and an aromatic dicarboxylic acid unit, which satisfies the features (a) and (b), will be specifically explained in detail.

In order to allow the liquid crystal polyester to exhibit Tm easily, the segment having high crystallinity may be introduced into the molecules of the liquid crystal polyester in a high percentage as described above. Specifically, the liquid crystal polyester is likely to exhibit Tm by:

-   (i) increasing the content of a structural unit derived from     p-hydroxybenzoic acid; -   (ii) increasing the content of 2,6-naphthalene units in the units;     and -   (iii) increasing the content of a segment including a copolymer of     an aromatic diol and an aromatic dicarboxylic acid polymerized     alternately. In other words, the structural unit derived from     p-hydroxybenzoic acid is likely to promote crystallinity of the     liquid crystal polyester, the 2,6-naphthalene units are likely to     promote crystallinity because of the so-called packing property to     be enhanced by the aromatic rings that stack one another and     densify, and the segment where an aromatic diol and an aromatic     dicarboxylic acid are copolymerized alternately is likely to promote     crystallinity since it increases regularity of the liquid crystal     polyester molecules. Any of these (i), (ii) and (iii) (hereinafter     referred to as “(i) to (iii)”) are advantageous in view of     exhibition of Tm.

In order to obtain the liquid crystal polyester exhibiting Tg easily, a monomer unit that provides the liquid crystal polyester molecules a flexural structure (hereinafter referred to as a “flexural monomer unit”) maybe introduced into the liquid crystal polyester molecules, among the above-mentioned monomer units. Specifically, when the flexural monomer unit is introduced in an amount of 20% by mole or more based on the entire monomer units, the resultant liquid crystal polyester is likely to exhibit Tg. The flexural monomer unit is exemplified by a monomer unit derived from phthalic acid and that derived from isophthalic acid among the aromatic dicarboxylic acid units, and exemplified by a monomer unit derived from resorcin among the aromatic diol units. The liquid crystal polyester having the total content of the flexural monomer units (copolymerization ratio) of 20% by mole or more can lower Tg. For the liquid crystal polyester used in the present invention, the monomer unit derived from phthalic acid and/or isophthalic acid is more preferred among these flexural monomer units.

Accordingly, it becomes possible to obtain the liquid crystal polyester which is preferably used in the present invention, by controlling Tm of the liquid crystal polyester based on the amount of the above-described (i) to (iii) and bonding groups (the above-described amide group and carbonate ester group) to be introduced, and by controlling Tg based on the copolymerization ratio of the flexural monomer units. In the copolymerization ratio, the aromatic hydroxycarboxylic acid unit accounts for 30 to 80% by mole, the aromatic diol unit accounts for 10 to 35% by mole, and the aromatic dicarboxylic acid unit accounts for 10 to 35% by mole (the total of aromatic hydroxycarboxylic acid unit, the aromatic diol unit and the aromatic dicarboxylic acid unit is defined as 100% by mole). The liquid crystal polyester with such a combination and abundance ratio of monomer units relatively easily controls Tg and Tm, and enables the production of the liquid crystal polyester fiber having a high strength by the following heat treatment.

Among these, the liquid crystal polyester, which includes a monomer unit derived from parahydroxybenzoic acid and/or 2-hydroxy-6-naphthoeic acid, i.e., the preferable aromatic hydroxycarboxylic acid unit, a monomer unit derived from a compound selected from 4,4′-dihydroxybiphenyl, hydroquinone, resorcin and 2,6-dihydroxynaphthalene, i.e., the preferable aromatic diol unit, and a monomer unit derived from a compound selected from terephthalic acid, isophthalic acid, phthalic acid and 2,6-naphthalenedicarboxylic acid, i.e., the preferable aromatic dicarboxylic acid unit, is an all-aromatic liquid crystal polyester containing no aliphatic group in the molecular chain, and thus it is likely to satisfy the feature (b) and has an advantage that the liquid crystal polyester itself is highly heat resistant.

The liquid crystal polyester made from a combination of the monomer units preferred in view of exhibition of Tg and Tm is obtained by controlling the amount of the respective monomers that induces the respective monomer units for satisfying the intended copolymerization ratio, followed by polymerization thereof.

The following method can be applied to produce the liquid-crystalline polymer used in the present invention.

First, prepolymers with a relatively low molecular weight are prepared by conducting a melt-polymerization of respective raw material monomers for the liquid-crystalline polymers. The prepolymers can be synthesized by the method in which the hydroxyl groups in the aromatic hydroxycarboxylic acids and/or the aromatic diols are acylated with acid anhydride to obtained acylated aromatic hydroxycarboxylic acids and/or acylated aromatic diols, and the transesterification between the acylated groups and the carboxyl groups in the aromatic dicarboxylic acids and/or the acylated aromatic hydroxycarboxylic acids is carried out in a molten state. Examples of such a melt-polymerization are disclosed in Japanese Patent Application Laid-open Publication Nos. 2002-220444 and 2002-146003. The prepolymers obtained thus preferably have the molecular weight of from 2000 to 60000.

Next, after the obtained first and second prepolymer are cooled to a room temperature respectively to prepare the solids thereof, the solid are subjected to crushing or the like to prepare the prepolymers with the powdery state such as powder and flake. The prepolymers are preferably prepared so as to have the average particle diameter of 1 mm or less, and more preferably to have the average particle diameter of 0.1 to 1 mm.

Then, the prepolymer is subjected to solid-phase polymerization, e.g., by heating the prepolymer in a powder form, for high molecular quantification of the liquid crystal polyester. By carrying out the solid-phase polymerization and promoting the high molecular quantification of the liquid crystal polyester, the flow initiation temperature and Tm can be raised. Particularly, it is preferred that the solid-phase polymerization be continued until the polymerization degree corresponds to 250° C. or higher in view of the flow initiation temperature of the liquid crystal polyester. Since the reaction conditions of the solid-phase polymerization and the reaction conditions for preparing the liquid crystal polyester having an intended flow initiation temperature differ depending on the kind of monomer units that compose the applied liquid crystal polyester, it is preferred to optimize the reaction conditions properly by conducting preliminary experiments or the like. It is also allowed to rely on such operations that the liquid crystal polyester in the middle of the solid-phase polymerization is sampled at every predetermined time and the flow initiation time is sought, and the solid-phase polymerization is terminated when the flow initiation time becomes 250° C. or higher.

<Liquid Crystal Polyester Fiber>

Next, the liquid crystal polyester fiber of the present invention and the production method thereof will be explained.

In order to obtain the liquid crystal polyester fiber, there can be applied a method including a spinning step of melt-spinning a liquid crystal polyester into a fiber form and a heat treatment step of heat-treating the liquid crystal polyester formed into the fiber in order to highly strengthen the fiber. Herein, the fiber-form liquid crystal polyester obtained through the spinning step is referred to as a “liquid crystal polyester fiber 1”, hereinafter.

In the spinning step, the liquid crystal polyester is first molten at a temperature equal to or higher than the flow initiation temperature. The upper limit of the temperature concerning the melting is adjusted depending on Tm of the liquid crystal polyester to be used. The temperature is preferably in the range of (Tm+50)° C. or lower, more preferably (Tm+30)° C. or lower. The molten liquid crystal polyester is discharged from a proper spinning nozzle to be formed into a fiber form, and cooled to form the liquid crystal polyester fiber 1. The resultant liquid crystal polyester fiber 1 is reeled off using a wind roll bobbin or the like to obtain the liquid crystal polyester fiber 1 continuously. The pore size of the spinning nozzle is generally about 0.05 to 1.0 mm, and preferably from 0.1 to 0.5 mm. The discharge rate from the spinning nozzle is generally from 1 to 40 g/min, and preferably from 10 to 30 g/min. The discharge rate is not limited, but a proper rate (that does not cause thread breakage) should be selected since an extremely low/high discharge rate may cause thread breakage upon melt-spinning.

By carrying out the spinning in this way, the liquid crystal polyester fiber 1 is obtained, in which the liquid crystal polyester is relatively well oriented and crystallized. However, in order to suppress the degradation of the liquid crystal polyester itself as much as possible, it is preferred that the melting time of the liquid crystal polyester be shorter and the transport time of the molten liquid crystal polyester (i.e., residence time of the molten liquid crystal polyester) be adjusted as low as possible. For the melt-spinning, a commercially available melt-spinning apparatus (e.g., Polymer mate V, a multifilament apparatus manufactured by Chubu Kagakukikai Seisakusyo) can be used. Using such a melt-spinning apparatus, the melting time of the liquid crystal polyester can be shortened, and the transport time for converting the molten liquid crystal polyester into a fiber form can be further shortened. Further, using a melt-spinning apparatus equipped with heating means just before the spinning nozzle, the molten liquid crystal polyester is immediately discharged through the spinning nozzle, and thereby both melting time and transport time can be shortened concomitantly.

Further, in order to provide higher orientation to the liquid crystal polyester composing the liquid crystal polyester fiber 1, it is preferred that the shear rate at the spinning nozzle be 10³ sec⁻¹ or higher, and the pore size of the spinning nozzle be smaller. In addition, when the fiber is reeled off using a wind roll bobbin, it is preferred that the take-up rate be higher. The number of pores of the spinning nozzle is not particularly limited, and can be selected properly depending on the kind of the melt-spinning apparatus to be used and a production volume to be needed.

It is also possible to add a light resistant agent, various particles such as carbon black and titanium oxide, colorants such as a pigment and a dye, an antistatic agent, an antioxidant and the like to the liquid crystal polyester upon melt-spinning, to the extent that the object of the present invention is not damaged markedly, and that the spinning property and the following heat treatment are not adversely affected in producing the liquid crystal polyester fiber 1.

The liquid crystal polyester fiber 1 thus prepared has a strength of about 5 to 8 (cN/dtex) as it is, so that the strength is relatively higher than that of organic fibers made from polyesters other than nylon and a liquid crystal polyester. However, by using the liquid crystal polyester that satisfies the features (a) and (b), the strength of the liquid crystal polyester fiber can be enhanced dramatically by the subsequent heat treatment.

In the heat treatment of the liquid crystal polyester fiber 1, any form can be applied, such as allowing the liquid crystal polyester fiber 1 discharged from the spinning cap to pass through a heating furnace, reeling off the liquid crystal polyester fiber 1 around a wind roll bobbin and heating the fiber together with the bobbin, or heating the liquid crystal polyester fiber 1 while drawing it out of the wind roll bobbin. Considering the handleability and productivity, it is preferred to employ the form of heating the fiber together with the bobbin.

As an ambient gas for the heat treatment, air or a mixture gas of oxygen and carbon dioxide can be used, let alone inert gases such as nitrogen and argon. However, since the liquid crystal polyester tends to be susceptible to hydrolysis, it is preferred that the ambient gas be dehumidified, and it is more preferred that the dew point of the ambient gas be −20° C. or lower, particularly preferably −50° C. or lower.

The treatment time for the heat treatment is generally from 1 to 20 hours or so, although it can be optimized depending on a treatment temperature and an intended property of the resultant liquid crystal polyester fiber. It is preferred that the treatment time be shorter in view of productivity and energy consumption. Typically, a multi-stage heat treatment is exemplified, wherein heat treatment is carried out at about 120 to 150° C. for 0.5 to 1 hour, followed by at 180 to 200° C. for 0.5 to 3 hours, and further at 210 to 230° C. for 1 to 5 hours.

By using the liquid crystal polyester satisfying the features (a) and (b) as the liquid crystal polyester fiber 1, it becomes possible to obtain a liquid crystal polyester fiber having a high strength, even by a heat treatment under a low temperature condition, in which the maximum temperature reached is about 230° C. as described above. In addition, the production method of the liquid crystal polyester fiber according to the present invention can prevent agglutination of single yarns effectively, as compared with conventional production methods using a heat treatment that requires a temperature condition of around the melting point of a liquid crystal polyester. Further, the present method contributes to energy saving, so as to be advantageous in view of production.

The liquid crystal polyester fiber of the present invention improves the strength dramatically after the heat treatment as compared to before the heat treatment. Specifically, it is possible to establish the relation T_(y)/T_(x)≧2, where T_(x) (cN/dtex) denotes the strength of the liquid crystal polyester fiber 1 and T_(y) (cN/dtex) denotes the strength of the liquid crystal polyester fiber after a heat treatment. Further, the liquid crystal polyester fiber thus subjected to the heat treatment can realize a high strength of 10 (cN/dtex) or more easily.

Thus, the method for producing a liquid crystal polyester fiber, which achieves extremely high strength while effectively preventing agglutination of single yarns by a heat treatment under a low temperature condition of 230° C. or lower, is something that was not attained easily by conventional production methods of a liquid crystal polyester fiber, and is based on the unique findings of the present inventors. The strength of the liquid crystal polyester fiber denotes a strength measured at room temperature (about 23° C.), a sample interval of 20 cm and a pulling rate of 20 cm/min, using an autograph (AG-1KNIS manufactured by Shimadzu Corp.)

Incidentally, the liquid crystal polyester fiber of the present invention may be a core-sheath type composite yarn, a bimetal type composite yarn, or a fiber prepared by complex spinning of Umishima type and separated type, or may be a microfiber.

Further, the cross-sectional shape of the fiber is not particularly limited, and conventionally known shapes, such as round, triangle, multilobal, flat and hollow shapes, can be widely used. The cross-sectional shape of the fiber is formed into an intended shape according to a shape of the spinning nozzle used in the spinning step.

The liquid crystal polyester fiber prepared in the present invention not only possesses a property of high strength, but also fully maintains the properties of the liquid crystal polyester itself, i.e., low water absorbency, low dielectric property, vibration damping, dimensional stability, heat resistance, chemical resistance and the like, and thus useful as marine materials such as a fishing net, a nylon string and a rope, reinforcements such as an optical fiber code and a printed substrate, rubber reinforcements such as a tire code and a belt, and reinforcements for plastics and concretes. It can also be used as clothing materials such as a protective clothing and gloves.

Further, the liquid crystal polyester fiber of the present invention can be twisted with a fiber made from other polymers to make a composite fiber, and used for various applications.

Further, the liquid crystal polyester fiber of the present invention can be processed into various forms, such as a filament yarn, a cut fiber, a spun yarn, a rope shaped product and clothes (e.g., woven, knitted and nonwoven fabrics). In these forms, the nonwoven fabric is formed into various sheets such as a dry-laid nonwoven fabric, needle felt, spunlace and spunbond, although the method is not particularly limited. For example, when a wet-laid nonwoven fabric is produced, it is preferred to use a cut fiber 5 dtex or less in single yarn diameter and 3 mm to 10 mm in fiber length. Particularly, since the fiber has a low dielectric property, it gives an excellent electric effect when formed into a nonwoven fabric, and thus preferably used as a prepreg. In this case, other fibers may be used in combination.

The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are to be regarded as within the spirit and scope of the invention, and all such modifications as would be apparent to one skilled in the art are intended to be within the scope of the following claims.

EXAMPLES

The present invention is described in more detail by following Examples, which should not be construed as a limitation upon the scope of the present invention.

Glass transition temperature (Tg), melting point (Tm) and flow initiation temperature of polymers (including prepolymers thereof) and strength (tensile strength) of polymer fibers were measured by the following methods, respectively. Glass transition temperature measurement and melting point measurement:

As for the polymer to be measured, a calorie profile was measured obtained under the conditions described above using the differential scanning calorimetry system “DSC 6200” (manufactured by Seiko Instruments Inc.). Based on the calorie profile, a galass transition temperature (Tg) and a melting point (Tm) of the polymer were obtained.

Flow Initiation Temperature:

Flow initiation temperature was measured with the use of a flow tester (CFT-500 type, manufactured by Shimadzu Corporation). Specifically, first, the polymer to be measure of about 2 g was filled in the capillary type rheometer which was equipped with the die of 1 mm in inside diameter and 10 mm in length. And, while the load of 9.8 MPa (100 kg/cm²) was added and the liquid-crystalline polymer was extruded from the nozzle at the temperature rising speed of 4° C./minute, the temperature when melting viscosity showing 4800 Pa·s (48000 poise) was measured, and this temperature was assumed to be the flow initiation temperature.

Tensile Strength:

Tensile strength of liquid crystal polyester fiber was measured using the autograph AG-1KNIS (manufactured by manufactured by Shimadzu Corporation) with sample interval of 20 cm and tensile rate of 20 cm/min. at a room temperature (of about 23° C.).

Synthesis Example 1

In a reactor equipped with a stirring device, a torque meter, a nitrogen gas introduction pipe, a thermometer, and a reflux condenser, 828 g (6.0 moles) of p-hydroxybenzoic acid, 559 g (3.0 moles) of 4,4′-dihydroxybiphenyl, 498 g (3.0 moles) of isophthalic acid and 1348 g (13.2 moles) of acetic anhydride were added, and these were stirred. Next, 0.19 g of 1-methylimidazol was added in the mixture after having been stirred. After the inside of the reactor was sufficiently substituted with nitrogen gas, the temperature in the mixture was raised up to 150° C. for 15 minutes under nitrogen gas flow, and the mixture was refluxed for one hour while maintaining the temperature. Then, the temperature was raised up to 320° C. over two hours and 50 minutes and was maintained at 320° C. for 75 minutes while removing the distillate of by-product acetic acid and unreacted acetic anhydride. Then, the contents were taken out.

Next, after being cooled to room temperature, the contents taken out above were crushed with a crusher to give the powder of the prepolymer having the particle diameter of about 0.1 mm to about 1 mm. A portion of the prepolymer was taken out to measure a flow initiation temperature of the prepolymer. As a result, the flow initiation temperature of the prepolymer was 230° C.

Then, after the prepolymer powder was heated from 25° C. to 200° C. over one hour, the powder was heated from this temperature to 232° C. over five hours, and further kept at the temperature for three hours to be polymerized in the solid phase. The powder after the solid-phase polymerization was cooled to give a liquid-crystalline polymer without oxidation deterioration. The flow initiation temperature of the obtained liquid-crystalline polyester was 258° C. Moreover, the DSC measurement of the liquid-crystalline polyester was conducted. As a result, the glass transition temperature was 128° C. and the melting point was 310° C. It is noted that the liquid crystal polyester has 50% by mole of the aromatic hydroxycarboxylic acid unit (derived from p-hydroxybenzoic acid), 25% by mole of the aromatic diol unit (derived from 4,4′-dihydroxybiphenyl) and 25% by mole of aromatic dicarboxylic acid unit (derived from isophthalic acid) based on the entire structural units in the polyester, which can be obtained from the amounts of the raw material monomers used. Also, it is noted that the liquid crystal polyester has 25% by mole of flexural monomer unit (derived from isophthalic acid)

Synthesis Example 2

A liquid-crystalline polyester was obtained in the same manner as in Synthesis Example 1 except that the temperature of the prepolymer powder in the solid-phase polymerization was changed from 232° C. to 220° C. The flow initiation temperature of the obtained liquid-crystalline polyester was 241° C. Moreover, the DSC measurement of the liquid-crystalline polyester was conducted. As a result, the glass transition temperature was 125° C. and the melting point was 305° C.

Synthesis Example 3

In a reactor equipped with a stirring device, a torque meter, a nitrogen gas introduction pipe, a thermometer, and a reflux condenser, 911 g (6.6 moles) of p-hydroxybenzoic acid, 409 g (2.2 moles) of 4,4′-dihydroxybiphenyl, 91 g (0.55 moles) of isophthalic acid, 274 g (1.65 moles) of terephthalic acid and 1235 g (12.1 moles) of acetic anhydride were added, and these were stirred. Next, 0.17 g of 1-methylimidazol was added in the mixture after having been stirred. After the inside of the reactor was sufficiently substituted with nitrogen gas, the temperature in the mixture was raised up to 150° C. for 15 minutes under nitrogen gas flow, and the mixture was refluxed for one hour while maintaining the temperature. Then, after 1.7 g of 1-methylimidazol was further added, the temperature was raised up to 320° C. over two hours and 50 minutes while removing the distillate of by-product acetic acid and unreacted acetic anhydride. The point of time when the rise of the torque was admitted was considered to be the end of the reaction, and the contents were taken out.

Using the contents, the powder of prepolymer (having the particle diameter of about 0.1 mm to about 1 mm) was obtained in the same manner as in Synthesis Example 1. A portion of the prepolymer was taken out to measure a flow initiation temperature of the prepolymer. As a result, the flow initiation temperature was 257° C.

Then, after the prepolymer powder was heated from 25° C. to 250° C. over one hour, the powder was heated from this temperature to 285° C. over five hours, and further kept at the temperature for three hours to be polymerized in the solid phase. The powder after the solid-phase polymerization was cooled to give a liquid-crystalline polymer. The flow initiation temperature of the obtained liquid-crystalline polyester was 327° C. Moreover, the DSC measurement of the liquid-crystalline polyester was conducted. As a result, the melting point was 340° C., while the glass transition temperature was not observed. It is noted that the liquid crystal polyester has 60% by mole of the aromatic hydroxycarboxylic acid unit (derived from p-hydroxybenzoic acid), 20% by mole of the aromatic diol unit (derived from 4,4′-dihydroxybiphenyl) and 20% by mole of aromatic dicarboxylic acid units (derived from isophthalic and terephthalic acids) based on the entire structural units in the polyester, which can be obtained from the amounts of the raw material monomers used. Also, it is noted that the liquid crystal polyester has 5% by mole of flexural monomer unit (derived from isophthalic acid).

Synthesis Example 4

In a reactor equipped with a stirring device, a torque meter, a nitrogen gas introduction pipe, a thermometer, and a reflux condenser, 941 g (5.0 moles) of 2-hydroxy-6-naphthoic acid, 273 g (2.5 moles) of 4-aminophenol, 415.3 g (2.5 moles) of isophthalic acid and 1123 g (11 moles) of acetic anhydride were added, and these were stirred. After the inside of the reactor was sufficiently substituted with nitrogen gas, the temperature in the mixture was raised up to 150° C. for 15 minutes under nitrogen gas flow, and the mixture was refluxed for three hour while maintaining the temperature. Then, the temperature was raised up to 320° C. over two hours and 50 minutes while removing the distillate of by-product acetic acid and unreacted acetic anhydride. The point of time when the rise of the torque was admitted was considered to be the end of the reaction, and the contents were taken out.

Using the contents, the powder of prepolymer (having the particle diameter of about 0.1 mm to about 1 mm) was obtained in the same manner as in Synthesis Example 1. A portion of the prepolymer was taken out to measure a flow initiation temperature of the prepolymer. As a result, the flow initiation temperature was 229° C.

Then, after the prepolymer powder was heated from 25° C. to 180° C. over one hour, the powder was heated from this temperature to 220° C. over five hours, and further kept at the temperature for three hours to be polymerized in the solid phase. The powder after the solid-phase polymerization was cooled to give a liquid-crystalline polymer. The flow initiation temperature of the obtained liquid-crystalline polyester was 272° C. Moreover, the DSC measurement of the liquid-crystalline polyester was conducted. As a result, the glass transition temperature was 129° C., while the melting point (Tm peak) was not observed. It is noted that the liquid crystal polyester has 50% by mole of the aromatic hydroxycarboxylic acid unit (derived from 2-hydroxy-6-naphthoic acid, 25% by mole of the unit of aromatic amine with hydroxyl group (derived from 4-aminophenol) and 25% by mole of aromatic dicarboxylic acid units (derived from isophthalic acid) based on the entire structural units in the polyester, which can be obtained from the amounts of the raw material monomers used. Also, it is noted that the liquid crystal polyester has 25% by mole of flexural monomer unit (derived from isophthalic acid).

Synthesis Example 5

A prepolymer was obtained in the same manner as in Synthesis Example 1 except that the period of time for maintaining the temperature of the prepolymer at 320° C. (while removing the distillate) was changed from 75 minutes to 65 minutes. The flow initiation temperature of thus-obtained prepolymer was 227° C.

Using the prepolymer, a liquid-crystalline polyester was obtained in the same manner as in Synthesis Example 1 except that the temperature of the prepolymer powder in the solid-phase polymerization was changed from 232° C. to 249° C. The flow initiation temperature of thus-obtained liquid-crystalline polyester was 266° C. Moreover, the DSC measurement of the liquid-crystalline polyester was conducted. As a result, the glass transition temperature was 128° C. and the melting point was 313° C.

Synthesis Example 6

A prepolymer was obtained in the same manner as in Synthesis Example 1 except that 1-methylimidazol was not added, that the period of time for distillation at 150° C. was changed from one hour to three hour, and that the period of time for maintaining the temperature of the prepolymer at 320° C. (while removing the distillate) was changed from 75 minutes to 85 minutes. The flow initiation temperature of thus-obtained prepolymer was 234° C.

Using the prepolymer, a liquid-crystalline polyester was obtained in the same manner as in Synthesis Example 1 except that the temperature of the prepolymer powder in the solid-phase polymerization was changed from 232° C. to 245° C. The flow initiation temperature of thus-obtained liquid-crystalline polyester was 267° C. Moreover, the DSC measurement of the liquid-crystalline polyester was conducted. As a result, the glass transition temperature was 128° C. and the melting point was 313° C.

Synthesis Example 7

A prepolymer was obtained in the same manner as in Synthesis Example 1 except that 1-methylimidazol was not added, that the period of time for distillation at 150° C. was changed from one hour to three hour, and that the period of time for maintaining the temperature of the prepolymer at 320° C. (while removing the distillate) was changed from 75 minutes to 90 minutes. The flow initiation temperature of thus-obtained prepolymer was 237° C.

Using the prepolymer, a liquid-crystalline polyester was obtained in the same manner as in Synthesis Example 1 except that the temperature of the prepolymer powder in the solid-phase polymerization was changed from 232° C. to 244° C. The flow initiation temperature of thus-obtained liquid-crystalline polyester was 267° C. Moreover, the DSC measurement of the liquid-crystalline polyester was conducted. As a result, the glass transition temperature was 128° C. and the melting point was 312° C.

Synthesis Example 8

A prepolymer was obtained in the same manner as in Synthesis Example 1 except that the period of time for maintaining the temperature of the prepolymer at 320° C. (while removing the distillate) was changed from 75 minutes to 90 minutes. The flow initiation temperature of thus-obtained prepolymer was 241° C.

Using the prepolymer, a liquid-crystalline polyester was obtained in the same manner as in Synthesis Example 1 except that the temperature of the prepolymer powder in the solid-phase polymerization was changed from 232° C. to 243° C. The flow initiation temperature of thus-obtained liquid-crystalline polyester was 268° C. Moreover, the DSC measurement of the liquid-crystalline polyester was conducted. As a result, the glass transition temperature was 128° C. and the melting point was 312° C.

Example 1

The liquid crystal polyester prepared in Synthesis Example 1 was granulated into pellets, which make it easier to form the polyester into a fiber in the spinning step. Next, using Polymer mate V, a multifilament apparatus manufactured by Chubu Kagakukikai Seisakusyo, the polyester was passed through a filter made of stainless steel to carry out melt-spinning at 310° C. The spinning cap used has a pore diameter of 0.3 mm and number of pores of 24, and the fiber was reeled off at a discharge rate of 25 g/min and a spinning rate of 400 m/min. The spun original yarn was wound around a metal bobbin, and subjected to a heat treatment in which the temperature at 150° C. was maintained for 1 hour, then was raised from 150° C. to 230° C. over 80 minutes, and then was again maintained at 230° C. for 5 hours, to obtain a heat-treated yarn. The evaluation results of the heat-treated yarn are shown in Table 1.

Example 2

Using the liquid crystal polyester prepared in Synthesis Example 2, a heat-treated yarn was obtained in the same manner as in Example 1 except that the condition of the heat treatment was changed to such a condition that the wound yarn was heated at 150° C. for 1 hour, then the heating temperature was raised from 150° C. to 220° C. over 70 minutes, and then the yarn was heated at 220° C. for 5 hours. The evaluation results of the heat-treated yarn are shown in Table 1.

Example 3

Using the liquid crystal polyester prepared in Synthesis Example 5, a heat-treated yarn was obtained in the same manner as in Example 1 except that the temperature at melt-spinning was changed from 310° C. to 315° C. The evaluation results of the heat-treated yarn are shown in Table 1.

Example 4

A heat-treated yarn was obtained in the same manner as in Example 3 except that the liquid crystal polyester prepared in Synthesis Example 6 was used instead of using the polyester prepared in Synthesis Example 5. The evaluation results of the heat-treated yarn are shown in Table 1.

Example 5

A heat-treated yarn was obtained in the same manner as in Example 3 except that the liquid crystal polyester prepared in Synthesis Example 7 was used instead of using the polyester prepared in Synthesis Example 5. The evaluation results of the heat-treated yarn are shown in Table 1.

Example 6

A heat-treated yarn was obtained in the same manner as in Example 3 except that the liquid crystal polyester prepared in Synthesis Example 8 was used instead of using the polyester prepared in Synthesis Example 5. The evaluation results of the heat-treated yarn are shown in Table 1.

Comparative Example 1

The liquid crystal polyester prepared in Synthesis Example 2 was granulated into pellets, which make it easier to form the polyester into a fiber in the spinning step. Next, using Polymer mate V, a multifilament apparatus manufactured by Chubu Kagakukikai Seisakusyo, the polyester was passed through a filter made of stainless steel to try to carry out melt-spinning at 300° C. The spinning cap used has a pore diameter of 0.3 mm and number of pores of 24. The discharge rate of polyester was 25 g/min. However, thread breakage was occurred upon melt-spinning, and therefore, a fiber of the liquid crystal polyester was not obtained.

Comparative Example 2

The liquid crystal polyester prepared in Synthesis Example 3 was granulated into pellets. Next, using a screw-type extruder, a melt-spinning was carried out at 370° C. The spinning cap used has a pore diameter of 0.2 mm and number of pores of 150, and the fiber was reeled off at a spinning rate of 600 m/min. The spun original yarn was wound around a metal bobbin, and subjected to a heat treatment at 150° C. for 1 hour, 150° C. to 230° C. for 80 minutes, and 230° C. for 5 hours, to obtain a heat-treated yarn. The evaluation results of the heat-treated yarn are shown in Table 2.

Comparative Example 3

Using the liquid crystal polyester prepared in Synthesis Example 4, a heat-treated yarn was obtained in the same manner as in Example 1 except that the discharge rate was changed from 25 g/min to 20 g/min and a spinning rate was changed from 400 m/min to 300 m/min. The evaluation results of the heat-treated yarn are shown in Table 2.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Tensile strength of polyester 5.6 5.6 5.3 6.9 6.2 5.4 fiber prior to heat treatment T_(X) (cN/dtex) Tensile strength of polyester 13.0 11.5 11.2 15.1 13.8 10.9 fiber after heat treatment T_(Y) (cN/dtex) T_(Y)/T_(X) 2.3 2.1 2.1 2.3 2.2 2.0

TABLE 2 Comparative Comparative Comparative Example 1 Example 2 Example 3 Tensile strength of polyester — 6.9 4.9 fiber prior to heat treatment T_(X) (cN/dtex) Tensile strength of polyester — 8.1 7.5 fiber after heat treatment T_(Y) (cN/dtex) T_(Y) /T_(X) — 1.2 1.5

It was found that the liquid crystal polyester fiber using the liquid crystal polyester, in which a glass transition temperature (Tg) and a melting point (Tm) are observed by a DSC measurement and the flow initiation temperature is 250° C. or higher, enhances the strength dramatically by a heat treatment under a temperature condition lower than conventional ones, and gives a liquid crystal polyester fiber having an extremely high strength (10 cN/dtex or more) after the treatment (Examples 1 to 6). On the other hand, it was impossible to form a fiber form in melt-spinning in the case of the liquid crystal polyester having a flow initiation temperature of lower than 250° C., even when using the same material monomer (Comparative Example 1). Further, the fiber formed of a liquid crystal polyester having either Tg or Tm not to be observed exhibited a strength inferior to that of the liquid crystal polyester in Example 1 (Comparative Examples 2 and 3). 

1. A method for producing a liquid crystal polyester fiber, the method comprising a step of spinning a liquid crystal polyester that satisfies the following features (a) and (b): (a) a glass transition temperature and a melting point are observed by differential scanning calorimetry, and (b) a flow initiation temperature is 250° C. or higher, which is followed by a heat treatment at 230° C. or lower.
 2. The production method according to claim 1, wherein the liquid crystal polyester is a liquid crystal polyester having a structural unit derived from a compound selected from phthalic acid and isophthalic acid in an amount of 20% by mole or more of the total structural units in the polyester.
 3. The production method according to claim 1, wherein the liquid crystal polyester is a liquid crystal polyester having a glass transition temperature of 150° C. or lower, which is determined by differential scanning calorimetry.
 4. The production method according to claim 1, wherein the liquid crystal polyester is a liquid crystal polyester having a melting point of 250° C. or higher, which is determined by differential scanning calorimetry.
 5. The production method according to claim 1, wherein the liquid crystal polyester fiber obtained after the heat treatment has double or more of strength compared to the strength of liquid crystal polyester fiber prior to the heat treatment.
 6. A liquid crystal polyester fiber obtained by the production method according to claim
 1. 7. The liquid crystal polyester fiber according to claim 6, the fiber having a tensile strength of 10 (cN/dtex) or higher when measured at a temperature of 23° C.
 8. A nonwoven fabric comprising the liquid crystal polyester fiber according to claim
 6. 